Sunan ad-Darimi

Explanation:
Because all these actions are contrary to cleanliness and proper etiquette.

Hadith Commentary:
➊
The intention of Imam Bukhari rahimahullah is that not only is it impermissible to perform istinja (cleansing after relieving oneself) with the right hand, but even touching the private part with the right hand while urinating should be avoided. The honor and status of the right hand demand that it be kept away from the private part in every situation and be used only for good deeds. The left hand should be employed for inappropriate and disliked actions.

➋
It was the noble habit of the Messenger of Allah sallallahu alayhi wa sallam that after urinating, he would perform ablution (wudu), and at times he would even drink the water left over from the ablution. We have been commanded to follow in his footsteps. Along with the etiquettes of relieving oneself, we have also been informed of the etiquettes of drinking, hence it is stated that one should not breathe into the vessel. This ruling is specific to drinking water, as clarified in the hadith: "When any of you drinks water from a vessel, he should not breathe into the vessel." (Fath al-Bari: 1/834)

➌
In the narration of Abu Dawud, it is mentioned that one should not drink water in a single breath. (Sunan Abi Dawud, Book of Purification, Hadith: 31) The narration of Sahih Bukhari requires that when a person drinks water, he should not breathe into the vessel, so that germs and the like, which are expelled through breathing, do not enter the water. The narration of Abu Dawud mentions another etiquette of drinking water: that one should not drink water in a single breath, because doing so is the habit of animals. Rather, one should quench one's thirst in three breaths. And Allah knows best.

1:
From this it is understood that for disliked actions, the left hand should be used so that the respect and dignity of the right hand remain preserved.

Benefits and Issues:
➊ When it is prohibited to touch or hold the private part with the right hand at the time of an important necessity such as istinja (cleaning oneself after relieving oneself), then one should be even more cautious in normal circumstances. Women are also bound by this same ruling.
➋ The Islamic etiquette for drinking something is that it should be drunk in three breaths.

24. Urdu Commentary:
➊ In this hadith, although the state of urination is mentioned, the ruling for the state of defecation is even more applicable, as it is stated in the hadith: «أو ان نستنجي بالیمین» “And he (sallallahu alayhi wa sallam) also forbade us from cleaning ourselves with the right hand.” [صحیح مسلم ، الطھارۃ ، حدیث : 262] And by “istinja” (cleaning oneself), what is specifically meant here is the removal of filth (najasa), particularly excrement (braz).
➋ It is necessary to protect the left hand from impurity because food and the like are primarily eaten with it, although incidentally the left hand may also be used alongside. Sometimes, it becomes necessary to take assistance from the left hand while eating.
➌ Although the hand that has come into contact with impurity becomes pure after washing, it is contrary to sound taste (zawq salim) to contaminate the hand used for eating with filth, to the extent that even the vessel used for the latrine and for ablution (wudu) are kept separate, even though rationally there is no difference. Thus, this issue is more a matter of natural disposition and taste than pure reason, and the Shari‘ah gives great consideration to sound taste (zawq salim) as well.

Urdu Commentary:
(1)
A distinctive feature of Islamic civilization is that it gives special importance to purity and cleanliness. In this regard, the etiquettes of istinja (cleansing after relieving oneself) have also been taught. In this hadith, the etiquette mentioned is that if there arises a need to touch the private parts, the right hand should not be used. Similarly, while performing istinja, the right hand should be kept away from impurity.

(2)
The distinction between the right and left hand is also among the etiquettes of Islamic civilization. The right hand is for those actions which are, according to the Shari‘ah, custom, or natural disposition, considered desirable, and the left hand is for those actions which are, according to custom or natural disposition, considered undesirable. Performing istinja is a human necessity; otherwise, by nature, a person does not like to touch the place of impurity. This is why the left hand has been designated for this purpose. In matters that are preferred, the Prophet (sallallahu alayhi wa sallam) would use the right hand and give preference to the right side. ‘Aishah (radi Allahu anha) narrates that the Messenger of Allah (sallallahu alayhi wa sallam) liked to begin with the right side in all his affairs, such as: performing ablution (wudu), combing his hair, and putting on his shoes. (Sahih al-Bukhari, Book of Ablution, Chapter: Beginning with the Right in Ablution and Ritual Bath, Hadith: 168; and Sahih Muslim, Book of Purification, Chapter: Beginning with the Right in Purification, etc., Hadith: 268)